Process and system for quality management and analysis of via drilling

ABSTRACT

A process for laser forming a blind via in at least one layer of a circuit substrate having a plurality of capture pads of varying geometry can include, for at least one blind via to be formed in at least one layer of a circuit substrate, evaluating a capture pad geometry value (such as area and/or volume) within a predetermined distance from a drilling location with respect to a blind via geometry value (such as area and/or volume) to be formed at the drilling location. The process can include setting at least one laser operating parameter based on the evaluation in order to obtain a desired capture pad appearance after blind via formation. The process can include imaging a capture pad area defined as an area within a predetermined distance from a blind via drilling location in at least one layer of a circuit substrate, quantifying at least one appearance value for the imaged capture pad area, and determining an acceptability of the imaged capture pad areas based on the quantified appearance value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/484,531, filed Jul. 11, 2006.

FIELD OF THE INVENTION

The present invention relates to a process and system for laser formingblind vias in at least one layer of a circuit substrate having aplurality of capture pads of varying geometry.

BACKGROUND OF THE INVENTION

The use of a pre-etched window as a mask for laser drilling blind viasin a multi-layer circuit board is generally known. Dense pin countand/or dense component placement on multi-layer circuit boards orpolymer based multi-chip modules can create an interconnect densityproblem referred to in the industry as “via starvation”. One solution tothe problem of “via starvation” is the formation of blind viasinterconnecting one or more layers in a multi-layer circuit board ormulti-chip module.

One quality criterion in blind via formation is copper capture padappearance. Capture pad appearance is sometimes referred to as “shiny”,“melted too much” or “dark”, and tends to be a very subjectivecriterion. Some customers specify the desired process parameter by thissubjective measure in addition to throughput considerations. It has beenobserved that laser operating parameters affect the appearance of thecopper capture pad. For high fluence process, the pad tends to be meltedshowing a “shiny” appearance. When the fluence is set very low, theappearance is somewhat “dark”. The number of pulses applied to thematerial can also affect the appearance. Another observation is that theappearance varies depending on the laser operating parameter orcharacteristics, such as pulse width. It has also been observed that theappearance varies depending on the capture pad geometry even when thesame laser processing parameters are used. When a variation in qualityis observed in the entire panel process, it is sometimes difficult toresolve the source of the quality variation, since data coming from thecomputer aided design (CAD) system relates only to the desired drillinglocation.

Ultraviolet (UV) laser and image projection processing has been used fordrilling blind via in integrated circuits (IC) packaging substrates. Thecurrent process typically applies a single set of laser operatingparameters to all vias or holes that are intended to yield the samegeometry between two layers in a printed circuit design. However, due tovarying capture pad geometry, the end results are sometimes less thandesirable. When applying a fixed set of laser operating parameters, theresults vary from “dark” copper when drilling in a solid copper plane,to delaminated copper on 110 micrometer (Mm) copper capture pads.

SUMMARY OF THE INVENTION

A process for laser forming a blind via in at least one layer of acircuit substrate having a plurality of capture pads of varying geometrycan include for at least one blind via to be formed in at least onelayer of a circuit substrate, evaluating a capture pad geometry value(such as area and/or volume) within a predetermined distance from adrilling location with respect to a blind via geometry value (such asarea and/or volume) to be formed at the drilling location. The processcan also include setting at least one laser operating parameter based onthe evaluation in order to obtain a desired capture pad appearance afterblind via formation.

A process for laser forming a blind via in at least one layer of acircuit substrate having a plurality of capture pads of varying geometrycan include imaging a capture pad area defined as an area within apredetermined distance from a blind via drilling location in at leastone layer of a circuit substrate, quantifying an appearance value forthe imaged capture pad area, and determining acceptability of the imagedcapture pad area based on the quantified appearance value.

Other applications of the present invention will become apparent tothose skilled in the art when the following description of the best modecontemplated for practicing the invention is read in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 is a simplified schematic view of a system for laser forming viasin at least one layer of a circuit substrate having a plurality ofcapture pads of varying geometry;

FIG. 2 is a detail of a CAD/CAM circuit layout pattern having aplurality of capture pads of varying geometry, where a blind via is tobe drilled in a center of a capture pad, and where a phantom circledefines a capture pad area within a predetermined distance from adrilling location;

FIG. 3 is a cross-sectional view taken as shown in FIG. 2 illustratingstacked vias and vias drilled in a plurality of capture pads of varyinggeometry;

FIGS. 4A through 4E illustrate a comparison of copper appearance indrilled blind via using different laser operating parameters, orapplying a fixed set of laser operating parameters to capture pads ofvarying geometry, where FIG. 4A corresponds to a subjective “shiny”surface texture, FIG. 4C corresponds to a subjective “matt” or “grainy”surface texture and FIG. 4E corresponds to a subjective “dark” surfacetexture; and

FIG. 5 illustrates a simplified schematic flow diagram of a process forlaser forming a blind via in at least one layer of a circuit substratehaving a plurality of capture pads of varying geometry.

DETAILED DESCRIPTION

Referring now to FIG. 1, a process or method for quality management andanalysis of blind via can include a CAD/CAM system 10 including circuitlayout design data. The CAD design data can include at least one layerof a circuit substrate having a plurality of capture pads of varyinggeometry including drilling locations and sizes for forming vias and/orblind vias therein. The CAD design data can be transferred by anysuitable means, schematically illustrated as arrow 12, to a laserprocessing system 14. Using the location of a via and/or a blind via tobe drilled and pad location and/or geometry as a reference image, laseroperating parameters can be set or associated with the CAD design datareceived from the CAD/CAM system in order to laser form a via or blindvia in at least one layer of the circuit substrate. The process can theninclude an imaging device or station 16 located in-line (or on-line) oroff-line, to analyze pad and via location to generate a quality index orappearance value. Quality index value information can be fed back to thelaser processing system 14 through any suitable means, schematicallyillustrated as arrow 18 for laser operating parameter verification,adjustment or optimization, and/or can be fed back to the CAD/CAM system10 through any suitable means, schematically illustrated as arrow 20,for verification, adjustment or optimization of the location/geometry offormed via and capture pads, and for mapping quality index or appearancevalues corresponding to each location/geometry.

It should be recognized that the present invention can be used forquality management and analysis and/or as a feedback signal to the priordesign and/or processing systems in order to verify and/or adjustcurrent locations, geometries and/or operating parameters in order toobtain a desired capture pad appearance after blind via formation.

Referring now to FIGS. 2 and 3, a CAD/CAM circuit layout pattern 22 isillustrated in detail where a plurality of capture pads 24 of varyinggeometry are to be formed with a blind via 26 in a drilling locationassociated with the capture pad 24. The outer phantom perimeter 28illustrates a radial distance or other predetermined distance foranalysis where copper capture pad appearance is not affected outside theboundary of perimeter 28. Copper pad geometry data, along with viadrilling size, can be used to evaluate a capture pad geometry valueconnected to a drilling location within a predetermined distance fromthe drilling location versus a blind via geometry value to be formed atthe drilling location. The evaluation can include value comparison,and/or lookup tables, and/or calculations, or the like.

By way of example and not limitation, a ratio of capture pad geometryvalue to blind via geometry value is calculated in order to rank theratios into predetermined ranges associated with the use of differentlaser processing parameters. The CAD system 10 sends blind via drilllocation/geometry information, capture pad location/geometry informationand/or pad/via geometry ratio or corresponding laser operatingparameters to the laser drill system 14. Alternatively, an evaluation ofthe capture pad geometry value with respect to the blind via geometryvalue is performed and/or the values are ranked by the laser drillsystem, so that different laser operating parameters can be applied.

Alternatively, the CAD system can segregate a geometry value, such as asingle-tool drill file, into a multi-tool drill file based on theevaluation of capture pad geometry value (such as area and/or volume)with respect to blind via geometry value (such as area and/or volume),as evaluated in the CAD system. The analysis can also include acomparison to previous drill steps to determine if stacked vias (such asthat illustrated to the left of FIG. 3) are present.

By way of example and not limitation, separate laser drill files aregenerated based on capture pad geometry value to blind via geometryvalue ratios ranked into different groups to set different laseroperating parameters for each group, such as a destination layer, laserfluence, number of pulses, pulse width, or any combination thereof. Byway of example and not limitation, stacked blind vias can be assigned adestination layer value of 14; ratios in the range of 1% to 13% areassigned a destination layer value of 15; ratios greater than 13% and upto 17% are assigned a destination layer value of 16; ratios greater than17% and up to 20% are assigned a destination layer value of 17; andratios greater than 20% and up to 100% are assigned a destination oflayer value of 18. A multi tool drill file is generated based on theratio of via geometry value (e.g. area or volume) versus capture padgeometry (e.g. area or volume) as evaluated in the CAD system, or in thelaser processing system, or any combination thereof.

The imaging device or station 16 can evaluate and quantify an appearanceof the capture pad after via formation. As seen in FIGS. 4A through 4E,when applying a fixed set of laser processing parameters, the appearanceof the capture pad can vary from “dark” copper when drilling in a solidcopper plane (illustrated in FIG. 4E) to delaminated copper on 110micrometer (μm) copper pads (illustrated in FIG. 4A). Copper appearancescan also vary in drilled blind via using different laser parameters. Forexample, the appearance illustrated in FIG. 4A is typically referred toby the descriptive subjective term “shiny” surface texture. The copperappearance illustrated in FIG. 4C is typically referred to by thedescriptive subjective term “matt” or “grainy” surface texture. Thecopper appearance illustrated in FIG. 4E is typically referred to by thedescriptive subjective term “dark” surface texture. FIG. 4B illustratesa copper appearance that falls between what is subjectively referred toas “shiny” and “matt” surface textures. FIG. 4D illustrates a copperappearance that falls between the subjective description of “dark” and“matt” surface textures.

The subjective character of the terms used to describe the coppercapture pad appearance after blind via formation has hindered qualitycontrol and processing control efforts. The present invention accordingto one embodiment uses the vision system 16 to quantify an appearance ofthe capture pad. This enables a user to reference copper appearance as anumber, rather than as merely a descriptive subjective term.

The process or method according to one embodiment of the presentinvention includes performing a histogram analysis and/or fractaldimension analysis to provide at least one numerical appearance value.For example, if the “matt” or “grainy” surface texture of a coppercapture pad is preserved during laser machining of a blind via, then thefractal dimension is high (salt and pepper effect), and the intensitydistribution in the histogram is bimodal with roughly equal area in eachintensity group. Accordingly, the properties of an image of a capturepad can be quantified by a number for fractal dimension and/or by anumber for the symmetry between two populations in the histogram. Forexample, a higher fractal dimension value corresponds to a “matt” or“grainy” surface texture of a copper capture pad, while a lower fractaldimension value corresponds to a “shiny” surface texture of a coppercapture pad. Also, a histogram value approximating unity (one) cancorrespond to a “matt” or “grainy” surface texture of a copper capturepad. In this case, for example, a histogram value less than unity (one)corresponds to a “shiny” surface texture or a copper capture pad, whilea histogram value greater than unity (one) corresponds to a “dark”surface texture of a copper capture pad.

The quantified numbers are transferred back to the laser processingsystem 14 and/or CAD/CAM system 10 to verify, adjust and/or optimize thelaser processing parameters in order to minimize the quality variance indrilling results over an entire circuit pattern. The CAD/CAM system cananalyze a relation between pad geometry and via quality with thequantified numbers from the vision imaging device or station 16. Thevision imaging device or a station 16 can also perform othermeasurements, such as top/bottom diameter and circularity measurements.

In operation, and referring now to FIG. 5, a process for laser forming ablind via in at least one layer of a circuit substrate having aplurality of capture pads of varying geometry includes, for at least oneblind via to be formed in at least one layer of a circuit substrate,evaluating a capture pad geometry value (such as area and/or volume)within predetermined distance from a drilling location with respect to ablind via geometry value (such as area and/or volume) to be formed atthe drilling location. Then, at least one laser operating parameter isset based on the evaluation in order to obtain a desired capture padappearance after blind via formation. The at least one laser processparameter to be set can be selected from a group consisting of laserfluence, number of laser pulses, laser pulse width and any combinationthereof. The process can optionally include comparing a given drilllayer to an adjacent drill layer to determine if blind vias are stackedone on top of another.

The process according to one embodiment of the present invention caninclude defining a drill layer, defining a scan area adjacent a drilllocation in the defined drill layer, defining a set of evaluationranges, wherein the evaluation ranges can include calculated ratiosdefined as a capture pad geometry value (such as area and/or volume)within a predetermined distance from a drilling location with respect toa blind via geometry value (such as area and/or volume) to be formed atthe drilling location, and selecting a destination layer for drill toolscorresponding to a particular set of ratio ranges.

A process according to one embodiment of the present invention caninclude imaging a capture pad area defined as an area within apredetermined distance from a blind via drilling location in at leastone layer of a circuit substrate, quantifying an appearance value forthe imaged capture pad area, and determining acceptability of the imagedcapture pad area based on the quantified appearance value. Theappearance value can be quantified as a numeric appearance value for theimaged capture pad area to minimize subjectivity in a quality judgmentof a laser-formed blind via in the imaged capture pad area. The numericappearance value can be obtained according to an embodiment of thepresent invention by performing a fractal dimension analysis on theimaged capture pad area to obtain a numeric value for fractal dimension,where a higher value corresponds to a subjective “matt” or “grainy”surface texture and a lower value correspondence to a subjective “shiny”surface texture for the imaged capture pad area. The numeric appearancevalue can be obtained according to an embodiment of the presentinvention by performing a histogram analysis on the imaged capture padarea to obtain a numeric value for symmetry between populations in thehistogram, where a value approximating unity (one) corresponds to asubjective “matt” or “grainy” surface texture, a value smaller thanunity (one) corresponds to a subjective “shiny” surface texture and avalue greater than unity (one) corresponds to a subjective “dark”surface texture of the imaged capture pad area. The numerical appearancevalue according to an embodiment of the present invention can includeperforming a histogram analysis and a fractal dimension analysis on theimaged capture pad area to quantify one or more numeric appearancevalues for the image capture pad area.

Acceptability of the imaged capture pad area according to an embodimentof the present invention can be based on the quantified appearancevalue. The process can include analyzing a relationship between capturepad geometry value (such as area and/or volume) with respect to blindvia geometry value (such as area and/or volume) forevaluation/comparison with respect to at least one appearance value fora plurality of imaged capture pad areas to minimize variance in qualityof laser formed blind via over an entire circuit pattern. At least onelaser processing parameter can be verified, adjusted, and/or optimizedbased on the relationship analyzed between the capture pad geometryvalue (such as area and/or volume) with respect to blind via geometryvalue (such as area and/or volume) and at least one appearance value fora plurality of captured pad areas with laser formed blind via located inthe imaged capture pad areas.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiments but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures as is permitted under the law.

What is claimed is:
 1. A process for laser drilling blind vias in atleast one layer of a circuit substrate having a plurality of capturepads of varying geometry, comprising: before blind via drilling,evaluating a relationship between a capture pad geometry value within apredetermined distance from a drilling location of a capture pad withrespect to a blind via geometry value for a blind via to be drilled atthe drilling location; and setting at least one laser operatingparameter based on a result of the evaluation in order to obtain adesired capture pad appearance after blind via drilling, wherein thedesired capture pad appearance is a subjective surface texture of thecapture pad after blind via drilling.
 2. The process of claim 1 whereinthe at least one laser process parameter to be set comprises at leastone of laser fluence, number of laser pulses and laser pulse width. 3.The process of claim 1, further comprising: comparing an anticipatedblind via location on a given drill layer to an anticipated blind vialocation on an adjacent drill layer to determine if blind vias arestacked from layer to layer.
 4. The process of claim 1, furthercomprising: analyzing a capture pad appearance after blind via drilling.5. The process of claim 1 wherein evaluating the capture pad geometryvalue comprises: calculating a ratio of the capture pad geometry valuewithin the predetermined distance from the drilling location withrespect to the blind via geometry value to be drilled at the drillinglocation for each blind via to be drilled in the at least one layer ofthe circuit substrate.
 6. The process of claim 1, further comprising:laser drilling the blind via in the capture pad using the at least onelaser operating parameter.
 7. The process of claim 1, furthercomprising: comparing an actual blind via location on a given drilllayer to an actual blind via location on an adjacent drill layer todetermine if blind vias are stacked from layer to layer.
 8. The processof claim 1 wherein setting at least one laser parameter based on aresult of the analysis of the capture pad appearance comprises:adjusting at least one laser process parameter based on analyzed capturepad appearance for a plurality of capture pad areas with drilled blindvias located in the plurality of capture pad areas.
 9. The process ofclaim 1 wherein setting at least one laser parameter comprises:adjusting at least one laser process parameter based on feedback from arelationship between a ratio of capture pad geometry value to blind viageometry value and an analyzed capture pad appearance for a plurality ofcapture pad areas with drilled blind vias located in the plurality ofcapture pad areas.
 10. The process of claim 9 wherein the at least onelaser process parameter to be set comprises at least one of laserfluence, number of laser pulses and laser pulse width.
 11. The processor claim 1 wherein the subjective surface texture of the capture pad isrepresented by a number.
 12. The process of claim 11 wherein the numbercorresponds to a numerical appearance value calculated by performing atleast one of a histogram analysis or a fractal dimension analysis of animage of the capture pad after blind via drilling.
 13. The process ofclaim 1 wherein analyzing a capture pad appearance after blind viadrilling comprises: imaging a capture pad area defined as an area withina predetermined distance from a blind via drilling location in at leastone layer of a circuit substrate; and quantifying an appearance valuefor the imaged capture pad area as a numerical appearance valueindicative of a subjective surface texture of the imaged capture padarea.
 14. The process of claim 13 wherein quantifying the appearancevalue comprises: performing a fractal dimension analysis on the imagedcapture pad area to obtain the numerical appearance value as a fractaldimension, wherein a level of the numerical appearance value correspondsto one of a plurality of subjective surface textures for the imagedcapture pad area.
 15. The process of claim 13 wherein quantifying theappearance value comprises: performing a histogram analysis on theimaged capture pad area to obtain the numerical appearance value as asymmetry between populations in the histogram, wherein the symmetrycorresponds to one of a plurality of subjective surface textures for theimaged capture pad area.
 16. The process of claim 13, furthercomprising: determining acceptability of the imaged capture pad areabased on the quantified appearance value.
 17. The process of claim 13wherein quantifying the appearance value comprises performing at leastone of a histogram analysis or a fractal dimension analysis of theimaged capture pad area.
 18. A process for laser forming a blind via inat least one layer of a circuit substrate having a plurality of capturepads of varying geometry, comprising: before blind via drilling and forat least one blind via to be drilled in at least one layer of a circuitsubstrate, evaluating a capture pad geometry value within apredetermined distance from a drilling location with respect to a blindvia geometry value to be drilled at the drilling location; and settingat least one laser operating parameter based on a result of theevaluation in order to obtain a desired capture pad appearance afterblind via drilling; and analyzing a capture pad appearance after blindvia drilling, wherein analyzing the capture pad appearance after blindvia drilling comprises: imaging a capture pad area defined as an areawithin a predetermined distance from a blind via drilling location in atleast one layer of a circuit substrate; quantifying an appearance valuefor the imaged capture pad area as a numerical appearance valueindicative of a subjective quality judgment of the imaged capture padarea; and determining acceptability of the imaged capture pad area basedon the quantified appearance value; and wherein quantifying theappearance value comprises: performing a fractal dimension analysis onthe imaged capture pad area to obtain the numerical appearance value asa fractal dimension, wherein a level of the numerical appearance valuecorresponds to a subjective surface texture for the imaged capture padarea.
 19. A process for laser forming a blind via in at least one layerof a circuit substrate having a plurality of capture pads of varyinggeometry, comprising: before blind via drilling and for at least oneblind via to be drilled in at least one layer of a circuit substrate,evaluating a capture pad geometry value within a predetermined distancefrom a drilling location with respect to a blind via geometry value tobe drilled at the drilling location; setting at least one laseroperating parameter based on a result of the evaluation in order toobtain a desired capture pad appearance after blind via drilling; andanalyzing a capture pad appearance after blind via drilling, whereinanalyzing the capture pad appearance after blind via drilling comprises:imaging a capture pad area defined as an area within a predetermineddistance from a blind via drilling location in at least one layer of acircuit substrate; quantifying an appearance value for the imagedcapture pad area as a numerical appearance value indicative of asubjective quality judgment of the imaged capture pad area; anddetermining acceptability of the imaged capture pad area based on thequantified appearance value; and wherein quantifying the appearancevalue comprises: performing a histogram analysis on the imaged capturepad area to obtain the numerical appearance value as a symmetry betweenpopulations in the histogram, wherein the symmetry corresponds to asubjective surface texture for the imaged capture pad area.